I've always noticed when Emma gets tired her eyes will be red at night - sometimes she also gets so tired she's like a little kid that refuses to settle down and acts like a maniac untill she passes out. I call them her devil eyes when she's doing it.
Henry though has two unpigmented haws (third eyelid) and so he almost always has red eyes. When he gets tired (and he's a puppy so he's often tired) his eyes will droop a little more and so they are very noticible.
I first looked it up when a family dog always had one red eye and learned about the unpigmented haw.
Missing Ellie, B:06/15/98 D:06/10/08 - Fawn, docked and floppy
Raising Emma, B:04/19/09 - Flashy Brindle, docked and floppy
& New baby Henry, B:02/14/12 - Flashy Brindle, docked and floppy